Circuit boards generally are comprised of alternating layers of copper or other conducting metal and woven fiberglass-epoxy material. Holes are drilled into these boards to provide passages between the surface and different interior layers. Plating then is done with copper to provide completed circuits between the surface and the interior. A number of drills have been developed for circuit board drilling, eg. U.S. Pat. No. 3,778,180 to Ostrom, No. 4,080,093 to Maier, and No. 4,231,692 to Brabetz et al.
In the past five years or so industry has taken to employing more aggressive drilling practices, increasing both the feed rate of the drill into the workpiece and also increasing the rotational speed of the drill. Industry also has taken to using drills comprised of harder materials. Such drills, usually of a harder grade of tungsten carbide, have the advantages that they stay sharp longer, have a reduced tendency to snap or break in half, and tend to stay straight or resist bending. However, such drills generally are less shock resistant and tend to chip easier, especially under conditions of more aggressive drilling.
In circuit board drilling, chipping, particularly of the outside corner of the cutting edge, is critical because it is that part of the drill which finishes-off, that is smooths, the hole wall surface. Chipping can result in spiral etchings forming in the hole surface which interfere with the coating process and result in imperfect circuits.
Standard drills tend to have a helix angle in the neighborhood of 35.degree. for more efficient removal of workpiece chips up and out of the hole. Because such an angle adjacent the tip of a drill is not the best for cutting, sometimes an angle grind or rake is made in the drill tip to provide a smaller angle better for cutting. This generally has required an additional operation during fabrication of the drill.
The aforementioned problems have not been addressed by the drills developed to date.